Web tension control system

ABSTRACT

The present system maintains and controls the tension or speed of a moving web over a preselected range of web speeds in the vicinity of the prime mover speed by means of a web draw roll driven by controlled overspeed and underspeed clutches operating off the prime mover. A balanced, undamped, low inertia, preloaded dancer assembly contacts the web and web tension upsets are reflected in movements of the dancer assembly. Such movements are detected by a transducer and applied to an electronic controller which provides a continuous, proportional output signal to either the overspeed or underspeed clutch. The clutch applies a drive torque or holdback torque component to the draw roll which changes the web tension as needed to reposition the dancer assembly to its reference position.

United States Patent [191 Meihofer Oct. 14, 1975 [54] WEB TENSIONCONTROL SYSTEM [75] Inventor: Edward F. Meihofer, Norfolk, Mass.

[73] Assignee: Butler Automatic, 1nc., Canton,

Mass.

[22] Filed: Mar. 4, 1974 [21] Appl. No.: 447,695

[52] US. Cl. 226/44 [51] Int. Cl. 8651-1 25/04 [58] Field of Search226/44, 25, ll, 37, 42

[56] References Cited UNITED STATES PATENTS 2,067,755 l/l937 Dyer 226/44X 3,404,820 10/1968 Marano 226/44 X 3,734,368 5/1973 Kudelski 226/44Primary ExaminerRichard A. Schacher Attorney, Agent, or F irmCesari andMcKenna [57] ABSTRACT The present system maintains and controls thetension or speed of a moving web over a preselected range of web speedsin the vicinity of the prime mover speed by means of a web draw rolldriven by controlled overspeed and underspeed clutches operating off theprime mover. A balanced, undamped, low inertia, preloaded dancerassembly contacts the web and web tension upsets are reflected inmovements of the dancer assembly. Such movements are detected by atransducer and applied to an electronic controller which provides acontinuous, proportional output signal to either the overspeed orunderspeed clutch. The clutch applies a drive torque or holdback torquecomponent to the draw roll which changes the web tension as needed toreposition the dancer assembly to its reference position.

21 Claims, 2 Drawing Figures US. Patent Oct. 14,1975 Sheet 1 of23,912,145 1 l WEB TENSION CONTROL SYSTEM BACKGROUND OF THE INVENTIONThis invention relates to a system for controlling the tension or speedin a continuous web moving to or from web processing apparatus.

Web tension may vary because of variations in the modulus of elasticityof the web along its length, changes in web thickness, speed of theweb-consuming machine and for other reasons.

In many instances, then, it is necessary to control the tension in amoving web as it is drawn into or out of a printing press, embosser,coater or other such machine through which the web must move undercontrolled tension to be processed properly.

A number of systems have been devised to provide this tension controland they often include a floating dancer assembly in contact with themoving web which changes position in response to web tension changes.Dancer position is then used to control the speed of a driven roll incontact with the web. The dancer is normally preloaded so that, inoperation, it tends to assume a selected reference position which isindicative of the desired web tension. In the case of the infeed device,any increase in tension causes the dancer to move in one direction fromits reference position. This movement initiates an increase in the speedof the driven roll which tends to decrease the tension in the web sothat the dancer returns to its reference position. On the other hand, adecrease in web tension causes the dancer to move in the oppositedirection from its reference position, producing a decrease in the speedof the driven roll so that web tension increases sufficiently to returnthe dancer to its reference position. Thus, any web tension upsets arereflected in movements of the dancer which, in turn, control the speedof the driven roll to compensate for the tension upsets.

The outfeed device operates in a comparable way to maintain uniformtension in web exiting such processing apparatus.

There are several known types of floating dancer tension controldevices. One type, shown in US. Pat. No. 3,087,663, requires a separateregulated d.c. motor driving a differential drive system to control thespeed of the driven roll to maintain proper web tension. The added motorand differential increase the initial cost of the system and its spacerequirements.

In another type of system we are aware of, the speed of the draw roll ischanged by over and underspeed clutches operating off the main lineshaft. The clutches are controlled by limit switches which sense extremepositions of the dancer. In response to a web tension increase, thedancer moves up until it trips one switch which then actuates theoverspeed clutch, resulting in an increase in the speed of the draw rollwhich lessens web tension so that the dancer moves down until it tripsthe other limit switch. This actuates the underspeed clutch, resultingin a decrease in speed of the draw roll and an increase in web tension,causing the dancer to travel upwards again, and so on.

While such systems work fairly well at slow web speeds, theirperformance at high speeds, i.e. on the order of 2,000 fpm and more, isnot satisfactory. When the dancer roll reaches the ends of its path oftravel, which may be as long as inches, it must immediately reversedirection. This is reflected in a change in the pressure in the dancerloading cylinder. At fast web speeds, these pressure pulses upset thestability of the system unless a large and expensive surge tank oraccumulator is incorporated into the system.

Additional instabilities are introduced into that type of prior infeedbecause it cannot compensate for changes in web tension on thedownstream side of the infeed. Even worse, it will translate any webspeed change on the downstream side into an unwanted tension change.Also, if the web slips on the draw roll as little as O. l%, controlproblems arise.

A variation of that type of infeed arrangement is shown in US. Pat. No.3,659,767. There, dancer position is used to control a variable ratiotransmission or variator coupled to the draw roll. While its performanceat high speeds is better than the arrangement just described, it stillis not entirely satisfactory. Stability problems still arise because ofload cylinder pulsing and the infeed is difficult to maintain in finetune. Further, the transmission is quite large and expensive. Also, thetransmission, being composed of many mechanical parts which must bemoved to change drive speed, responds too slowly. Further, the systemtends to'control web speed over a particular narrow range of, say,one-half percent. This causes various parts in the transmission tobecome worn or develop flats so that the system can no longer maintaincontinuous control over speed. Rather, it must, in effect,over-compensate in order to make the transmission respond beyond thatvery narrow speed range.

Thus, for these reasons and others, prior tension control apparatuswhich sense web tension and respond by changing the speed of the drawroll to restore the proper tension value do not operate entirelysatisfactorily in many applications.

SUMMARY OF THE INVENTION Accordingly, the present invention aims toprovide a web tension control system which maintains accurate controlover the tension in a moving web over a wide range of web speeds, e.g.from 50 fpm to 2,500 fpm and higher.

A further object of the invention is to provide a web tension controlsystem which is relatively inexpensive to make, maintain and operate.

Another object is to provide a web tension control system which correctsfor even small tension upsets.

A further object of the invention is to provide a system of this generaltype which maintains uniform web tension over a wide range of tensions.

A further object is-to provide a web tension control system which cancompensate for downstream changes in web tension and speed.

Yet another object of the invention is to provide a system of this typewhich is easily constructed from standard electromechanical components.

A further object of the invention is to provide a web tension controlsystem which is quite stable in operation even at high web speeds.

Yet another object is to provide a system of this type which isrelatively efficient as compared with those requiring separate controlmotors because energy is coupled back into the mechanical drive systemrather than being dissipated.

Other objects will in part be obvious and will in part appearhereinafter.

The invention accordingly comprises the features of construction,combination of elements and arrangement of parts which will beexemplified in the construction hereinafter set forth and the scope ofthe invention will be indicated in the claims.

For purposes of discussion, we will describe the present system in termsof an infeed controlling the tension of web entering a press, forexample. It should be understood, however, that it is equally applicableas an outfeed device and, indeed, in any application where it isdesirable to maintain uniform tension and/or speed in a moving web.

Briefly, web from a roll or other source passes over a driven roll andthence under a guide roll to the press which normally runs at a uniformspeed. A balanced, low inertia, undamped, floating roll dancer assemblyis positioned in the web path between the driven roll and guide roll.The dancer assembly is comprised of a pair of parallel rolls whichengage the web on opposite sides thereof and which are spaced from eachother in the direction of web travel. The rolls are rotatively mountedin a frame which, in turn, pivots on low friction bearings about an axiswhich is parallel to the axes of rotation of the rolls.

The dancer assembly is suitably preloaded so that it assumes a positionof maximum web storage. During operation of the system, when the web isunder tension, the dancer assembly tends to assume a reference positionmidway between its maximum and minimum storage positions.

Any tension change in the web downstream from the driven roll causes thedancer assembly to move from its reference position to accommodate thetension change. These dancer movements give rise to correspondingelectrical signals which cause a controller to vary the torque couplingin a pair of variable slip over and underspeed clutches connectedbetween the machine line shaft and the driven roll.

Since the dancer assembly is balanced, no preloading is required tocompensate for the weight of the dancer assembly. This is important whenit is necessary to process web under low tension because ten pounds ofloading pressure just to raise the dancer may represent as much as fiftypounds of web tension. Also, the assemblys small mass and low frictionmounting means that the dancer has relatively low mechanical inertia.Thus, it can change position very quickly in response to a web tensionchange to develop the proper corrective signal for the controller.

Also, as will be described later, the dancer assembly is speciallydesigned so that its movements do not affect the relationship betweenthe torques applied to the assembly by the tensioned web and the loadingdevice.

The controller develops electrical signals corresponding to both dancerposition and dancer velocity. These signals are summed and the resultantsignal is applied to a lag circuit which compensates for the smallamount of mechanical inertia in the system. The output of the lagcircuit is applied to continuously control the overspeed and underspeedclutches modulating between them, depending upon whether drive torque orholdback torque must be applied to the draw roll in order to restore theproper web tension that will cause the assembly to return to, and remainin, its reference position.

In other words, if there is a momentary tension increase in the web, thedancer assembly pivots so as to shorten the web path between the drivenroll and the guide roll. This dancer movement produces a correspondingelectrical signal which causes the controller to increase the torquecoupling in the overspeed clutch, thereby increasing the driving torqueon the driven roll. The tension in the web beyond the driven roll isthus decreased, as is the torque component on the dancer assembly due toweb tension, sufficiently to return the dancer assembly to its referenceposition.

Conversely, when the web suffers a tension decrease, the dancer assemblypivots so as to lengthen the path of the web between the driven roll andthe guide roll. This dancer movement, in turn, causes the controller toincrease the torque coupling in the underspeed clutch so that thenecessary holdback torque is applied to the driven roll to increase webtension just enough to return the dancer assembly to its referenceposition. In other words, the present system senses the tension in theweb and responds to a change therein by continuously modulating thetorque on the driven roll, rather than its speed, to effect tensioncorrection. This torque control is maintained over a predetermined rangeof web speeds in the vicinity of the speed of the line shaft, e.g. i 4%.The system can tolerate web slippage, web elongation, and evendownstream web speed changes over this preselected speed range. Anyenergy savings during torque modulation is coupled back into the machineline shaft so that the rating of the drive motor can be kept to aminimum.

The controller responds to both dancer position and rate so that itexercises very fast and continuous control over the torque applied tothe driven roll. Any change in that torque is reflected almostimmediately (i.e. five Hz or less) in a corrective change in web tensionthat will tend to return the dancer to its reference position.

Moreover, the subject dancer assembly in conjunction with the electroniccontroller and over and underspeed drives constitutes a true integratingtype of system. As a consequence, even a small change in dancer positiondue to a small tension upset results in a corrective torque componentbeing applied to the driven roll. Finally, since the system respondsquickly and is quite stable and exercises control only over therelatively short preselected web speed range, the excursions of thedancer assembly about its reference position are very short (e.g. 1/16inch) and web storage provided by only the two offset dancer rolls isquite adequate even at high web speeds.

Thus, the present arrangement provides continuously variable torquecontrol over the draw roll up to the full rating of the clutches tomaintain a constant trim on web tension from a web-up speed as low as 50fpm to normal operating speeds as high as 2500 fpm. Actually, thetension can be maintained to an accuracy of as high as 1% and even lowerover the entire web speed range.

BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of thenature and objects of the invention, reference should be had to thefollowing detailed description taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a diagrammatic view of the web tension con trol systemembodying the principles If this invention; and

FIG. 2 is a schematic diagram showing in greater detail certain elementsof the FIG. 1 system.

- DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of thedrawings, since this is a balanced system, the web can travel in eitherdirection through the system. However, in the infeed applicationspecifically shown, the web W is traveling from left to right. Web istrained around a driven draw roll and thence passed through a dancerassembly indicated generally at 12. From there, it is trained under aguide roll 14 and drawn into the web-consuming machine (not shown). Inresponse to tension changes in the web W, the dancer assembly 12 movesfrom its reference position to lengthen or shorten the web path asnecessary to maintain uniform tension in the web.

The dancer motion is detected by a control section shown generally at 16which responds by applying a drive torque or a holdback torque to thedraw roll 10. That, in turn, decreases or increases the tension in theweb W entering the dancer assembly as needed to return the dancerassembly to its reference position.

The draw roll 10 has a shaft 10a which is coupled to the control section16. Web slippage relative to roll 10 is minimized by means of a nip roll18 which is supported via its shaft 18a at the ends of a pair ofelongated end plates 22. The opposite ends of plates 22 are connected bypivots 24 to the machine frame (not shown). Nip roll 18 is pressureloaded against roll 10 by suitable means illustrated herein as pressurecylinders 26 acting between end plates 26 and the machine frame.

The dancer assembly 12 is comprised of a pair of parallel rolls 32 and34 which are spaced apart in the direction of web travel and engage webW on opposite sides thereof. The opposite ends of rolls 32 and 34 arejournaled in a pair of end plates 36a and 36b. A pair of stub shafts 38aand 38b project laterally from the centers of plates 36a and 36b,respectively, and these shafts are journaled in a pair of bearingassemblies 42a a 42b, respectively.

The components of the dancer assembly 12 are arranged with respect tothe draw roll 10 and guide roll 14 so that it is a completely balancedsystem. In other words, when the assembly is in its neutral position,the stretch of web between draw roll 10 and roll 32 is parallel to thestretch of web between roll 34 and guide roll 14. Both of these areperpendicular to the plane defined by the dancer rolls when the assemblyis in its reference (e.g. horizontal) position. When the dancer assemblypivots on its bearing assemblies 42a, 42b in one direction or the otherfrom its reference position, it either lengthens or shortens the path ofthe web W between rolls 10 and 14. Since the dancer is balanced, noloading is required to compensate for the weight of the dancer and,therefore, the system can operate at very low web tensions.

The dancer assembly is biased toward its web storage position bysuitable means illustrated herein as a loading cylinder 44. The end ofcylinder 44 is provided with an ear 46 which is connected by a pivot 48to an car 52 attached to the machine frame. The loading cyclinder shaft44a is pivotally connected at 49 to one of the dancer assembly endplates, i.e. plate 36a. The pivotal connection 49 between shaft 44a andthe end frame is situated outboard of shaft 38a so that the cylinder canimpart an appreciable torque to the dancer assembly.

Fluid under pressure is fed to cylinder 44 by way of a pressureregulator 50 and the cylinder pressure is adjusted so that the torqueexerted by the cylinder on the dancer assembly just offsets the opposingtorque due to the desired tension in web W so that the dancer assemblytends to assume a reference position as seen in FIG. 1. Of course, ifneed be, a duplicate loading cylinder can be stalled at the oppositeside of the assembly, i.e. at plate 361), to minimize racking of theassembly. Also, the loading cylinder 44 could just as well be arrangedto push down on plate 36a (and/or 36b) on the other side of bearingassembly 42a, (42b).

The present dancer assembly is specially designed so that no changes inweb tension arise because of changes in the relationship between thetorques on the assembly due to web tension and cylinder 44 at differentangular positions of the assembly. More particularly, one of theshortcomings of many prior dancer roll systems stems from the fact thatas the dancer assembly moves from its reference position, the forces onthe dancer due to web tension and dancer loading vary as a function ofthe angle to which the dancer assembly pivots due to a tension upset.Consequently, in those prior systems, the dancer assembly itselfintroduces unwanted tension changes in the web which adversely affectthe stability of the overall system. The dancer assembly shown in FIG. 1maintains constant web tension with a constant loading force fromcylinder 44 through all positions of the dancer assembly and there is noload cylinder pulsing when the dancer changes direction. This isaccomplished by arranging the loading cylinder 44 and its shaft 44g orother loading devices so that the force is always applied parallel tothe stretches of web W entering and leaving assembly 12. This conditionprevails provided the triangle defined by points A, B and C in FIG. 1 isgeometrically similar to the triangle defined by points A, D and E inthat figure. It can easily be shown mathematically that as long as theabove similarity persists, the force on the dancer assembly fromcylinder 44 will equal the product of a constant and the force on thedancer due to web tension for all deviations of the dancer assembly fromits neutral position.

Still referring to FIG. 1, any rotation of the dancer assembly on shafts38a and 38b is sensed by a suitable detector illustrated herein as apotentiometer 56 which is connected electrically to the control section16. More specifically, the potentiometer 56 applies a signal to anelectronic controller 58 in section 16 which corresponds to thedeviation of the dancer assembly from its illustrated referenceposition. The controller 58, in turn, provides an output to a variableslip overspeed clutch 62 or a variable slip underspeed clutch 64 coupledto draw roll 10, depending upon whether drive torque or holdback torqueshould be applied to the roll to correct web tension.

The clutches 62 and 64 are driven by the machine line shaft 66. Forthis, a toothed pulley 68 rotating with the line shaft 66 is connectedby a timing chain 72 to a toothed pulley 74 affixed to the input shaft62a of clutch 62 and also to a toothed pulley 76 connected to the inputshaft 64a of clutch 64. The pulley 74 has fewer teeth than pulley 68 andpulley 76 has correspondingly more teeth than pulley 68 to obtain thedesired speed differential between the two clutches over the desiredspeed range of, say i4% of the line shaft speed. The two clutches alsohave output shafts 62b and 64b which carry toothed pulleys 78 and 82,respectively, which are connected by a timing chain 84 to a toothedpulley 88 on the end of the draw roll shaft 10a. The clutches are set tomagnetically couple a selected torque to the draw roll 10 as needed tomaintain the dancer assembly in its reference position. The torquecoupling changes required to compensate for web tension variations aremade by controlling the current in the clutch coils. A variable slipmagnetic particle clutch suited for this purpose is manufactured by W.J. Industries Inc. and the Vickers Division of Sperry Rand Corp.although other comparable electric, pneumatic or friction clutches canalso be used, as long as they permit continuously varying control overthe torque applied to the draw roll.

The dancer assembly 12 and control section 16 together function as anerror position integrator so that the infeed responds to even very smalltension changes. Also, the control section 16 includes feedback betweenthe clutches and the controller and within the controller itself to keepinstabilities within the system to a minimum. As a consequence, theinfeed maintains very close control over web tension i.e. within 1% ofthe desired tension. Also, the excursions of the dancer assembly 12 fromits reference position are very small, i.e. on the order of 1/16 inchduring normal operation and only 1 to 2 inches during an emergency stopsituation. Since the dancer and overall system have very fast responseand only controls web speed within 14% of line speed, the dancerassembly does not require much storage capacity even at web speeds inexcess of 2500 fpm.

Turning now to FIG. 2, the controller 58 contributes substantially tothe basic stability of the overall system. It includes a regulated, dualpolarity power supply 82 connected to the potentiometer 56. The outputof the potentiometer, taken from the wiper 56a thereof, is applied to adc amplifier 84 and to a differentiator 86.

When the torque applied to the dancer assembly 12 (FIG. 1) by theloading cylinder 44 exactly offsets the torque due to the desired webtension and the dancer assembly assumes its horizontal referenceposition,, the voltage at the potentiometer 56 is 0 volts. However, whenthe dancer assembly moves in response to a tension change in the web, asignal is applied to amplifier 84 and differentiator 86.

The output of the amplifier is a signal corresponding to the position ofthe dancer, while the output of the differentiator is proportional tothe dancer velocity. These two signals are summed and applied to a lagcircuit 88 illustratively consisting of a high gain amplifier 89 and anRC circuit 92 connected between the amplifier 89 input and output. Thelag circuit is basically an integrator and provides a negative phaseshift to compensate for the mechanical inertia that remains in theinfeed system.

The circuit 88 output is applied directly to a diode 94 and by way of aninverter 96 to a diode 98, both diodes being arranged to pass onlypositive going signals. When the dancer assembly moves from itsreference position, a positive going signal is applied either to diode94 or diode 98, depending upon the direction of dancer movement. Forpurposes of discussion, we will assume that an increase in web tensionwhich causes the dancer assembly 12 to rotate clockwise in FIG. 1produces a positive going signal at diode 94.

The output of diode 94 is applied by way of a voltage regulator 102 toan overspeed firing circuit 104. The output of circuit 104 is used totrigger a pair of SCRs 106 and 108 connected to pass current duringalternate half cycles from a center tapped transformer 110 to a seriescircuit consisting of the coil 62c of overspeed clutch 62 and a resistor112. The output of the regulator 102 determines the firing angle of theSCRs and therefore controls the current in the clutch coil. A diode 111is connected across coil 62c to accomodate back EMF due to a collapsingfield in coil 62c. The voltage at the junction of coil 62c and resistor112 which is proportional to the current through the coil 620 is fedback and summed with the signal from diode 94. Thus, the signal appliedby voltage regulator 102 to firing circuit 104 makes the current throughthe clutch coil 620 at all times proportional to the output of the lagcircuit 88. Thus, very fast control is exercised over the overspeedclutch 62 in response to dancer movements due to a web tension increase.

Still referring to FIG. 2, similar circuitry follows the diode 98 tocontrol the underspeed clutch 64. Thus, the diode 98 output is appliedvia a voltage regulator 114 to an underspeed firing circuit 116 whoseoutput is used to trigger a pair of SCRs 118 and 122. These SCRs arearranged to conduct current from a transformer 124 during alternate halfcycles to a series circuit consisting of the underspeed clutch coil 64cand a resistor 126, with a diode 127 in parallel with the coil. Theoutput of the lag circuit also controls the firing angle of these SCRsand thus the current through coil 640. A voltage proportional to thecurrent through the coil 64c appears at the junction of that coil andthe resistor 126 and is fed back to the input of regulator 114 where itis summed with the signal from diode 98. Thus, the signal applied to thefiring circuit 116 controls the current through the underspeed clutchcoil 640 so that it is always proportional to the output of the lagcircuit 88. Resultantly, very close control is maintained over thetorque coupling between the underspeed clutch 64 and the draw roll 10 inresponse to counterclockwise dancer movement indicative of a web tensiondecrease.

In summary, then, the present system controls web tension by sensing webtension changes using a balanced, specially loaded, undamped dancerhaving low mechanical inertia and thus fast response to tension upsets.The dancer motion is sensed and a signal is applied to a controllerwhich continuously develops proportional control signals for thecontinuously controllable overspeed and underspeed clutches. Thus, theseclutches are modulated so as to apply just the proper torque componentto the draw roll to maintain the dancer in its reference position.

Since the clutches are set to always drive the draw roll at speedsslightly less than or slightly more than line shaft speed, theexcursions of the dancer are quite small, e.g. 1/16 inch, and systemoperation is quite stablefAlso, there is no dancer loading cylinderpulsing problem. Thus, web tension can be controlled with great accuracyover the full range of web speeds of from 50 fpm to 2500 fpm. As notedabove, in a typical case, the clutches are set to run 4% underspeed and4% overspeed and this amount may vary in different applications. As ageneral rule, however, it has been found that if the speed difference ismuch less than i2% of line shaft speed, tension upsets take a fairlylong time to correct. Also, if the difference is much more than 10% ofshaft speed, there is relatively large energy dissipation incident totension control.

The present system can accommodate downstream web speed changes becausethe clutches operate off the main line shaft, as does the downstreamwebconsuming machine so they compensate for any such change. Further,any speed upsets are not translated into web tension upsets as is thecase with some prior infeeds.

The present infeed is much less expensive than prior systems, costingonly about one-half as much as the type shown in U.S. Pat. No. 3,087,663and on the order of one-third less than the type depicted in said U.S.Pat. No. 3,659,767 and its mechanical clutch-type drive is only on theorder of one-third the size of the differential drives and motors inthose prior infeeds, yet it is easier to operate and control.

It will thus be seen that the objects set forth above are efficientlyattained. It should also be understood that certain changes may be madein the above description without departing from the scope of theinvention. For example, using this equipment, one can control draw rollspeed to impart a precise amount of elongation to a web by maintaining aprecise underspeed torque. This is useful, for example, to orientpolyethylene. As another example, in those applications such ascommercial printing where the tension in the web downstream from theinfeed is always greater than the tension in the web upstream therefrom,the overspeed clutch 62 and related control elements may be omitted.Conversely, in outfeed applications where the upstream web is undergreater tension than the downstream web, the underspeed components ofthe system may be dispensed with.

Also, the same principle can be applied to accurately control web speed.This involves sensing web speed changes instead of tension using a rolltachometer as the input to the controller. I

It will also be understood that the following claims are intended tocover all of the generic and specific features herein described.

I claim:

1. A system for controlling tension in a moving web comprising A. a drawroll for engaging said web,

B. a guide roll spaced from and parallel to the draw roll for engagingsaid web,

C. a balanced, undamped, low inertia dancer assembly positioned betweenthe draw roll and the guide roll, said dancer assembly including 1. aframe, and 2. a pair of spaced-apart dancer rolls rotatively supportedby the frame so that their axes are parallel to the draw and guide rollaxes and for engaging opposite sides of said web,

D. means for pivotally mounting the frame so that it can pivot about anaxis midway between and parallel to the dancer rolls in response tochanges in the tension of the web,

E. means for applying a torque to the dancer assembly so as to offsetthe torque applied thereto by a selected tension in the web so theassembly tends to assume a reference position, said torqueapplying meansapplying torque by exerting a force on said assembly which is directedalong a line which remains parallel to said entering and leaving webpaths as the dancer assembly deviates from its reference position,

F. means for detecting movement of the dancer assembly away from itsreference position in response to a change in web tension and producingan output signal in response thereto, and

G. means responsive to said output signal for applying the proper torquecorrection to the draw roll to change the tension in the web such thatthe dancer assembly tends to return to its reference position.

2. The system defined in claim 1 wherein the mounting means includebearing units which allow the assembly to pivot with minimum frictionallosses.

3. The system defined in claim 1 wherein the draw roll, guide roll anddancer rolls are arranged so that the web paths entering and leaving thedancer assembly are parallel.

4. The system defined in claim 3 wherein said entering and leaving webpaths are perpendicular to the plane defined by the dancer rolls whenthe dancer assembly is in its reference position.

5. The system defined in claim 1 and further including A. a nip roll,and

B. means for mounting the nip roll so as to form a nip with the drawroll to minimize slippage of web on the draw roll.

6. The system defined in claim 1 wherein the torque applying meanscomprises A. a pressurizable loading cylinder having a movable shaft,

B. a stationary support,

C. means for pivotally connecting the loading cylinder and its shaftbetween the frame and the support so that when the cylinder ispressurized, the frame is pivoted about its axis so as to lengthen theweb path between the draw roll and the guide roll, and

D. means for pressurizing the cylinder so as to maintain the assembly inits reference position at said selected web tension.

7. The system defined in claim 6 wherein the cylinder and its pivotalconnections to the assembly are arranged and adapted so that the axis ofthe cylinder always remains parallel to the entering and leaving webpaths as the assembly deviates from its reference position in responseto web tension changes.

8. The system defined in claim 1 wherein the detecting means include apotentiometer whose resistance changes as the assembly deviates from itsreference position.

9. The system defined in claim 1 wherein the torque correction applyingmeans comprise A. rotary drive means,

B. one of a controllable overspeed and underspeed clutch coupled betweenthe drive means and the draw roll, and

C. an electronic controller responsive to the output of the detectingmeans to provide control signals for the clutches so that said propertorque correction is applied to the draw roll.

10. The system defined in claim 9 wherein A. the other of the overspeedand underspeed clutch is also coupled between the drive means and thedraw roll,

B. the clutches are current controlled, and

C. the controller comprises 1. a first current regulator responsive tothe output of the detecting means for controlling the current in theoverspeed clutch,

2. a second current regulator responsive to the output of the detectingmeans for controlling the current in the underspeed clutch, and

3. means for coupling the output of the detecting means alternatively tothe overspeed and underspeed current regulators so that said propertorque correction is applied to the draw roll.

11. The system defined in claim wherein the coupling means include A.means for amplifying the output of the detecting means,

B. means for differentiating the output of the detecting means, and

C. means for summing the outputs of the amplifying and differentiatingmeans so that the signal applied to the current regulators reflects theinstantaneous position and velocity of the dancer assembly.

12. The system defined in claim 11 wherein the coupling means alsoinclude a lag circuit connected between the summing means and thecurrent regulators.

13. The system defined in claim 10 and further including A. means forsensing the current in the overspeed clutch and developing a firstsignal in response thereto,

B. means for sensing the current in the underspeed clutch and developinga second signal in response thereto,

C. means for summing the first signal at the input of the first currentregulator, and

D. means for summing the second signal at the input of the secondcurrent regulator, so as to provide negative feedback to more closelyregulate the currents in the overspeed and underspeed clutches.

14. A system for maintaining uniform tension in a moving web comprisingA. a draw roll for engaging the web,

B. a dancer assembly positioned adjacent the draw roll, said dancerassembly including 1. at least one dancer roll rotatively supportedparallel to the axis of the draw roll and for engaging the moving web,

2. means for mounting the dancer roll so that it can pivot about apivotal axis which is parallel to the axis of the draw roll in responseto changes in the tension of the web engaged by the dancer roll, and

3. means for applying a torque to the dancer assembly at said pivotalaxis so as to offset the torque applied thereto by a selected tension inthe web engaged by the dancer roll so that the assembly tends to assumea reference position about said pivotal axis,

C. means for detecting movement of the dancer assembly from itsreference position in response to a change in web tension and producingan output signal in response thereto,

D. drive means connected to the draw roll for applying tension to theweb,

E. a controllable, variable slip overspeed clutch connected to the drivemeans to provide a drive torque to the drive means,

F. a controllable, variable slip underspeed clutch connected to thedrive means to provide a holdback torque to the drive means, and

G. an electronic controller connected between the detecting means andthe clutches for controlling the clutches in accordance with the outputsignal from the detecting means so that the draw roll controls thetension in the web as needed to maintain the dancer assembly in itsreference position.

15. The system defined in claim 14 wherein the controller is comprisedof A. a position amplifier connected to amplify the output signal fromthe detecting means,

B. a differentiator connected to differentiate the output signal fromthe detecting means,

C. means for summing the outputs of the amplifier and differentiator soas to produce a signal representing the instantaneous position andvelocity of the dancer assembly,

D. means for controlling the slippage in said clutches in accordancewith the output of the summing means so that the torque applied by theclutches t0 the drive means is continuously trimmed.

16. The system defined in claim 15 and further including an adjustablelag circuit connected between the summing means and the controllingmeans for imparting a negative phase shift to the signal from thesumming means.

17. The system defined in claim 15 wherein the controlling means includemeans for applying the output signal from the summing meansalternatively to said clutches.

18. A system for controlling tension in a moving web comprising A. adriven roll for engaging the web,

B. a prime mover,

C. a continuously controllable clutch whose torque coupling varies inresponse to a control signal connected between the prime mover and thedriven roll, said clutch being arranged to operate at maximum torquecoupling at a speed slightly different from the speed of the primemover,

D. means for sensing a tension change in the web and developing acontrol signal corresponding thereto, and

E. means for coupling the control signal to the clutch to vary thetorque coupling from the prime mover to the driven roll to produce acompensating tension change in the web.

19. The system defined in claim 18 wherein A. the clutch iscurrent-controlled, and

B. the coupling means include a current regulator responsive to saidcontrol signal for controlling the current in the clutch.

20. The system defined in claim 19 and further including A. a secondcurrent controlled clutch,

B., a second current regulator responsive to the output of the sensingmeans for controlling the current in the second clutch,

C. means for applying said control signal alternatively to the first andsecond current regulators so that the proper torque is coupled to thedriven roll.

21. The system defined in claim 18 wherein said slightly different speedis within the range of i 2% to 10% of the speed of said prime mover.

1. A system for controlling tension in a moving web comprising A. a drawroll for engaging said web, B. a guide roll spaced from and parallel tothe draw roll for engaging said web, C. a balanced, undamped, lowinertia dancer assembly positioned between the draw roll and the guideroll, said dancer assembly including
 1. a frame, and
 2. a pair ofspaced-apart dancer rolls rotatively supported by the frame so thattheir axes are parallel to the draw and guide roll axes and for engagingopposite sides of said web, D. means for pivotally mounting the frame sothat it can pivot about an axis midway between and parallel to thedancer rolls in response to changes in the tension of the web, E. meansfor applying a torque to the dancer assembly so as to offset the torqueapplied thereto by a selected tension in the web so the assembly tendsto assume a reference posItion, said torque-applying means applyingtorque by exerting a force on said assembly which is directed along aline which remains parallel to said entering and leaving web paths asthe dancer assembly deviates from its reference position, F. means fordetecting movement of the dancer assembly away from its referenceposition in response to a change in web tension and producing an outputsignal in response thereto, and G. means responsive to said outputsignal for applying the proper torque correction to the draw roll tochange the tension in the web such that the dancer assembly tends toreturn to its reference position.
 2. The system defined in claim 1wherein the mounting means include bearing units which allow theassembly to pivot with minimum frictional losses.
 2. a pair ofspaced-apart dancer rolls rotatively supported by the frame so thattheir axes are parallel to the draw and guide roll axes and for engagingopposite sides of said web, D. means for pivotally mounting the frame sothat it can pivot about an axis midway between and parallel to thedancer rolls in response to changes in the tension of the web, E. meansfor applying a torque to the dancer assembly so as to offset the torqueapplied thereto by a selected tension in the web so the assembly tendsto assume a reference posItion, said torque-applying means applyingtorque by exerting a force on said assembly which is directed along aline which remains parallel to said entering and leaving web paths asthe dancer assembly deviates from its reference position, F. means fordetecting movement of the dancer assembly away from its referenceposition in response to a change in web tension and producing an outputsignal in response thereto, and G. means responsive to said outputsignal for applying the proper torque correction to the draw roll tochange the tension in the web such that the dancer assembly tends toreturn to its reference position.
 2. means for mounting the dancer rollso that it can pivot about a pivotal axis which is parallel to the axisof the draw roll in response to changes in the tension of the webengaged by the dancer roll, and
 2. a second current regulator responsiveto the output of the detecting means for controlling the current in theunderspeed clutch, and
 3. means for coupling the output of the detectingmeans alternatively to the overspeed and underspeed current regulatorsso that said proper torque correction is applied to the draw roll. 3.means for applying a torque to the dancer assembly at said pivotal axisso as to offset the torque applied thereto by a selected tension in theweb engaged by the dancer roll so that the assembly tends to assume areference position about said pivotal axis, C. means for detectingmovement of the dancer assembly from its reference position in responseto a change in web tension and producing an output signal in responsethereto, D. drive means connected to the draw roll for applying tensionto the web, E. a controllable, variable slip overspeed clutch connectedto the drive means to provide a drive torque to the drive means, F. acontrollable, variable slip underspeed clutch connected to the drivemeans to provide a holdback torque to the drive means, and G. anelectronic controller connected between the detecting means and theclutches for controlling the clutches in accordance with the outputsignal from the detecting means so that the draw roll controls thetension in the web as needed to maintain the dancer assembly in itsreference position.
 3. The system defined in claim 1 wherein the drawroll, guide roll and dancer rolls are arranged so that the web pathsentering and leaving the dancer assembly are parallel.
 4. The systemdefined in claim 3 wherein said entering and leaving web paths areperpendicular to the plane defined by the dancer rolls when the dancerassembly is in its reference position.
 5. The system defined in claim 1and further including A. a nip roll, and B. means for mounting the niproll so as to form a nip with the draw roll to minimize slippage of webon the draw roll.
 6. The system defined in claim 1 wherein the torqueapplying means comprises A. a pressurizable loading cylinder having amovable shaft, B. a stationary support, C. means for pivotallyconnecting the loading cylinder and its shaft between the frame and thesupport so that when the cylinder is pressurized, the frame is pivotedabout its axis so as to lengthen the web path between the draw roll andthe guide roll, and D. means for pressurizing the cylinder so as tomaintain the assembly in its reference position at said selected webtension.
 7. The system defined in claim 6 wherein the cylinder and itspivotal connections to the assembly are arranged and adapted so that theaxis of the cylinder always remains parallel to the entering and leavingweb paths as the assembly deviates from its reference position inresponse to web tension changes.
 8. The system defined in claim 1wherein the detecting means include a potentiometer whose resistancechanges as the assembly deviates from its reference position.
 9. Thesystem defined in claim 1 wherein the torque correction applying meanscomprise A. rotary drive means, B. one of a controllable overspeed andunderspeed clutch coupled between the drive means and the draw roll, andC. an electronic controller responsive to the output of the detectingmeans to provide control signals for the clutches so that said propertorque correction is applied to the draw roll.
 10. The system defined inclaim 9 wherein A. the other of the overspeed and underspeed clutch isalso coupled between the drive means and the draw roll, B. the clutchesare current controlled, and C. the controller comprises
 11. The systemdefined in claim 10 wherein the coupling means include A. means foramplifying the output of the detecting means, B. means fordifferentiating the output of the detecting means, and C. means forsumming the outputs of the amplifying and differentiating means so thatthe signal applied to the current regulators reflects the instantaneousposition and velocity of the dancer assembly.
 12. The system defined inclaim 11 wherein the coupling means also include a lag circuit connectedbetween the summing means and the current regulators.
 13. The systemdefined in claim 10 and further including A. means for sensing thecurrent in the overspeed clutch and developing a first signal inresponse thereto, B. means for sensing the current in the underspeedclutch and developing a second signal in response thereto, C. means forsumming the first signal at the input of the first current regulator,and D. means for summing the second signal at the input of the secondcurrent regulator, so as to provide negative feedback to more closelyregulate the currents in the overspeed and underspeed clutches.
 14. Asystem for maintaining uniform tension in a moving web comprising A. adraw roll for engaging the web, B. a dancer assembly positioned adjacentthe draw roll, said dancer assembly including
 15. The system defined inclaim 14 wherein the controller is comprised of A. a position amplifierconnected to amplify the output signal from the detecting means, B. adifferentiator connected to differentiate the output signal from thedetecting means, C. means for summing the outputs of the amplifier anddifferentiator so as to produce a signal representing the instantaneousposition and velocity of the dancer assembly, D. means for controllingthe slippage in said clutches in accordance with the output of thesumming means so that the torque applied by the clutches to the drivemeans is continuously trimmed.
 16. The system defined in claim 15 andfurther including an adjustable lag circuit connected between thesumming means and the controlling means for imparting a negative phaseshift to the signal from the summing means.
 17. The system defined inclaim 15 wherein the controlling means include means for applying theoutput signal from the summing means alternatively to said clutches. 18.A system for controlling tension in a moving web comprising A. a drivenroll for engaging the web, B. a prime mover, C. a continuouslycontrollable clutch whose torque coupling varies in response to acontrol signal connected between the prime mover and the driven roll,said clutch being arranged to operate at maximum torque coupling at aspeed slightly different from the speed of the prime mover, D. means forsensing a tension change in the web and developing a control signalcorresponding thereto, and E. means for coupling the contRol signal tothe clutch to vary the torque coupling from the prime mover to thedriven roll to produce a compensating tension change in the web.
 19. Thesystem defined in claim 18 wherein A. the clutch is current-controlled,and B. the coupling means include a current regulator responsive to saidcontrol signal for controlling the current in the clutch.
 20. The systemdefined in claim 19 and further including A. a second current controlledclutch, B. a second current regulator responsive to the output of thesensing means for controlling the current in the second clutch, C. meansfor applying said control signal alternatively to the first and secondcurrent regulators so that the proper torque is coupled to the drivenroll.
 21. The system defined in claim 18 wherein said slightly differentspeed is within the range of + or - 2% to 10% of the speed of said primemover.